1. Field of the Invention
The present invention relates to network interface apparatus for connecting an incoming coaxial transmission line to a subscriber coaxial transmission line and protecting the coaxial transmission lines against overvoltage conditions using a coaxial gas discharge tube surge arrestor.
2. Discussion of the Relevant Art
It is known in the prior art to use a network interface apparatus to connect incoming coaxial cable television signals to a subscriber's television set using modules with coaxial connectors ("coax modules") and to include in the apparatus individual telephone subscriber line modules for connecting telephone company and subscriber telephone lines. See, for example, U.S. Pat. No. 5,394,466 issued to Schneider et al. on Feb. 28, 1995. See also Square D Company brochure 1220BR9202R3/92, which also discloses that the coax modules are grounded by being screwed to a metal plate which is connected to earth ground at the time of installation. However, the known coaxial network interface apparatus do not provide a coaxial transmission line surge arrestor to protect the coaxial transmission lines from overvoltage conditions. Nor do they provide overvoltage protection for the telephone company and subscriber lines.
Gas discharge tubes suitable for use with conventional telephone lines are shown in Napiorkowski U.S. Pat. No. 4,212,047 issued on Jul. 8, 1980. However, such surge arrestors are unsuitable for coaxial transmission lines which have unique characteristics and requirements. Several attempts have, however, been made to provide gas discharge tube surge arrestors for coaxial transmission lines.
Kawanami U.S. Pat. No. 4,544,984 issued Oct. 1, 1985 (Kawanami '984) discloses a gas discharge tube surge arrestor for a coaxial transmission line. According to the Kawanami '984 patent, conventional gas discharge tubes, while suitable as surge arrestors for telephone lines, cannot be used for high frequency coaxial transmission lines because (1) the gas discharge tube has a considerable amount of capacitance and (2) the nature of the required connection is such that it greatly changes the impedance of the coaxial transmission line and causes reflections in the transmission line. According to the Kawanami '984 patent, there has previously been no surge arrestor which could be used in a high frequency coaxial transmission line (column 1, line 57 to column 2, line 4).
The Kawanami '984 patent discloses a surge arrestor which connects a gas discharge tube between the inner and outer conductors of the coaxial transmission line in a direction orthogonal to the direction of signal transmission. The unwanted increased capacitance associated with the use of a gas discharge tube in a coaxial transmission line is compensated for by reducing the effective cross sectional area of the inner conductor at the place where the gas tube contacts the inner conductor by cutting out a portion of the center conductor to create a flat area on which the gas tube rests.
Kawanami U.S. Pat. No. 4,509,090 issued on Apr. 2, 1985 (Kawanami '090) also explains why conventional gas discharge tubes have not been successfully employed as surge arrestors in coaxial transmission lines and discloses the same type of structure disclosed in the Kawanami '984 patent, i.e., a device which connects the gas discharge tube between the inner and outer conductors of the coaxial transmission line in a direction orthogonal to the direction of signal transmission. In FIG. 7 the Kawanami '090 patent provides information concerning the impact of reducing the effective cross sectional area of the center conductor at the place where it contacts the gas discharge tube, showing that small dimensional changes on the order of 1 or 2 millimeters have a significant effect on the voltage standing wave ratio (VSWR).
Mickelson U.S. Pat. No. 4,633,359 issued on Dec. 30, 1986 also discloses a surge arrestor for a coaxial transmission line in which a gas discharge tube is connected between the inner and outer conductors of the transmission line in a direction orthogonal to the direction of signal transmission. The asserted advantage of the Mickelson device is that it is "simpler and less expensive to fabricate." Like the Kawanami '090 and '984 patents, Mickelson uses a center conductor which is flattened at the place where the gas tube contacts the center conductor. In addition to serving as a seat for the gas tube, this flat area adjusts the inductance of the center conductor to compensate for the distributed capacitance of the gas tube. Chamfers are provided adjacent the flat area to match the impedance of the surge arrestor to that of the transmission line. It is well known that maximum power transfer occurs when matched impedances are employed.
Cook GB 2,083,945A discloses a coaxial transmission line gas discharge tube surge arrestor comprising a center electrode 7, a cylindrical outer electrode 1 and insulating ends 3 and 5. The center conductor can be "cranked" as shown in FIG. 2. A similar coaxial transmission line surge arrestor is shown in DE 3,212,684A1.
The present invention provides a new and improved gas discharge tube surge arrestor for coaxial transmission lines comprising a cylindrical conductive housing, an axial center conductor and insulating end caps, the ratio of the inner diameter of the conductive housing to the outer diameter of the center conductor being varied within the surge arrestor to match the impedance of the surge arrestor to that of the coaxial transmission line. The coaxial surge arrestor of the present invention is sufficiently small that it can be incorporated within or made an integral part of existing coaxial connectors. Further, the present invention results in a much simpler, easier to manufacture and, therefore, less expensive device. At the same time, the present invention permits compensating for the unwanted capacitance introduced by the presence of a gas discharge tube in the coaxial transmission line and further permits matching the impedance of the surge arrestor to that of the coaxial transmission line so as to provide a device having a useful frequency range extending from 50 MHz to at least 1 GHz.
Therefore, it is an object of the present invention to provide a network interface apparatus with a coaxial surge arrestor which has a characteristic impedance similar to that of the coaxial transmission line.
It is another object of the present invention to provide a network interface apparatus with a coaxial surge arrestor which permits compensating for the unwanted capacitance introduced by the use of a gas discharge tube in a coaxial transmission line.
It is another object of the present invention to provide a network interface apparatus with a coaxial surge arrestor which may be mounted within conventional coaxial cable components and which may be readily installed in existing coaxial transmission lines.
It is another object of the present invention to provide a network interface apparatus with a coaxial surge arrestor which includes fail safe protection so that overheating of the gas discharge tube will short the center conductor to ground, thereby protecting the equipment to which it is connected.
It is still another object of the present invention to provide a network interface apparatus with a coaxial surge arrestor which includes current limiting and/or low voltage protection.
It is another object of the invention to provide a network interface apparatus which splits the incoming coaxial transmission line into two or more subscriber coaxial transmission lines and provide a coaxial surge arrestor for the incoming and subscriber lines.
It is another object of the invention to provide a network interface apparatus with a coaxial surge arrestor, a module for interconnecting telephone company and subscriber lines and overvoltage protection for the telephone lines.